Cells 4

Question 1

CHROMOSOMES

Answer 1

Chromosomes carry the hereditary material –DNA bound with basic proteins (called histones).
Individual chromosomes are not visible in a non-dividing cell-instead a thread-like structure is observed the chromatin. In a dividing cell individual chromosomes are visible due to supecoiling
The chromosome number varies from one species to another but is always the same for normal individuals of the same species. Humans possess 46 chromosomes in all their somatic cells.

Question 2

Cell cycle

Answer 2

The cell life undergoes a regular pattern of events known as the cell cycle. It is
divided into 3 basic parts:
1. INTERPHASE: the longest period in a cell’s cycle. It is further divided into G1 phase= cell growth takes place here and much protein synthesis, more organelles and more cytoplasm are produced
S phase= where DNA replication takes place, and
G2 phase= where the cell prepares for mitosis
2. MITOSIS: during this period the NUCLEUS divides into two nuclei, each
contains the SAME number of chromosomes, and
3. CYTOKINESIS: where the cytoplasm and eventually the cell divides giving two
cells, each contains one nucleus

Question 3

Stages of Mitosis:

Answer 3

a) PROPHASE: the nuclear envelope disappears. The chromatin condenses and
individual chromosomes become visible. Centrioles migrate to the poles of the cell, creating a structure which occupies the entire cell called the SPINDLE. It is created by MICROTUBULES.
b) METAPHASE: The chromosomes are now fully visible and they are oriented on the equator of the spindle.
c) ANAPHASE: Centromeres divide and chromatids are pulled on top of the fibers of the spindle towards the opposite poles of the cell sliding on the microtubules.
d) TELOPHASE and CYTOKINESIS: The chromosomes de-condense. The nuclear envelopes reappear surrounding the chromosomes in the two poles of the cell. The spindle is destroyed. Cytokinesis in animal cells starts with constriction of the cytoplasm which divides the cell in two through a cleavage furrow. In plant cells a new cell wall (called cell plate) is created in the equator which splits the cell in two.

Question 4

Mitotic index formula

Answer 4

Mitotic index=𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑒𝑙𝑙𝑠 𝑠h𝑜𝑤𝑖𝑛𝑔 𝑚𝑖𝑡𝑜𝑠𝑖𝑠/number of cells counted

Question 5

Significance of mitosis

Answer 5

Through mitosis daughter cells are produced which are exact copies (clones) of the parent cell (contain exactly the same genetic material). This is important for:
• GROWTH: An organism will grow through repeated mitotic divisions of pre-existing cells
• REPAIR: Damaged cells are replaced by exact copies of the originals. By this way a damaged tissue may be repaired
• ASEXUAL REPRODUCTION: Lower organisms multiple through division without the existence of gametic cells and production of a zygote.

Question 6

CYCLINS AND CELL CYCLE CONTROL

Answer 6

Cyclins are a group of proteins which are used to ensure that the cell cycle proceeds at a correct time and that the cell only moves to the next stage of the cell cycle only when it is appropriate.
Cyclins are working by binding to enzymes called cyclin-dependent kinases. Active kinases attach phosphates to proteins which in turn are used for various events during the cell cycle, such as microtubule formation and chromatin remodeling.Unless specific cyclins reach a threshold concentration, the cell does not progress to the next stage of the cell cycle. Cyclins ensure the cell divides when new cells are needed and not at other times

Question 7

CANCER

Answer 7

In all types of cancers a population of cells multiplies in an unregulated manner, independent of normal control mechanisms. This causes the creation of a tumor.
Tumor formation is called oncogenesis. The process starts with mutations in genes which control the cell cycle called oncogenes. Mutations have to occur in many oncogenes in the same cell for control to be lost. There is a small chance for this to occur, however, since the body contains billions of cells, the overall risk is significant.
Chemical substances that increase the risk of mutations are called mutagens. Ionizing and UV radiation can also cause mutations and therefore tumors.
Primary tumors occur when a mass of cells is initially produced from uncontrolled divisions.
BENIGN TUMORS: They do not have the ability to transport into other parts of the body and then reside in the position of origin. These are primary tumors.
MALIGNANT TUMORS: They have the ability to metastasise and create other tumors into distant organs. The new tumor which is produced in distant organs is called a secondary tumor.